1. Field of the Invention
This invention relates to the wind drift problems facing an instrument pilot who attempts to execute a holding pattern at a radio fix. The device is intended to provide the best available holding pattern wind drift correction for virtually any situation that might be encountered by a wide range of aircraft.
2. Description of the Prior Art
An aircraft on an instrument flight plan, due to localized traffic congestion, may be required by Air Traffic Control (ATC) to establish an oval-racetrack-type holding pattern at a given ground-based radio fix 41 (see FIG. 3). These instructions are normally given a few minutes before the aircraft reaches the designated fix, allowing the pilot ample time to determine the proper entry to the pattern as depicted in the Federal Aviation Administration's (FAA) Airman's Information Manual (AIM). When the pilot reaches the fix, he or she is expected to execute 1 of 3 different types of entries (based on the aircraft's heading on entry relative to the holding pattern) to attempt to intercept the inbound (IB) leg approximately 1 minute's flight from the station or fix. Upon crossing the fix again, he is expected to execute a standard 42 (right turns) or non-standard 43 (left turns) holding pattern, as instructed by ATC, and to adjust subsequent holding patterns according to previous errors caused by the wind.
When crossing the fix without having been given clearance beyond that fix, the pilot should begin the outbound (OB) turn 44, or in other words, the turn to the OB leg 45. The pattern turns are expected to be flown at a standard turn rate of 3.degree. per second (2 minutes for a full 360.degree. turn) which can be determined by timing across 30.degree. of turn or by using a turn coordinator, a required instrument for flight under instrument flight rules (IFR). Without wind, the pilot, upon reaching the reciprocal (i.e., opposite direction) of the IB leg, would roll out of the turn and hold that direction for 1 minute. With no wind correction needed, the IB turn 46 would bring the plane back onto the IB leg 47 with a minute left to reach the fix again. The IB leg is always defined by a radial from a ground-based V.O.R. station, a final approach radio "beam" originating from what's called a localizer, or when using an Automatic Direction Finder in conjunction with a ground-based Non-Directional Beacon, a magnetic bearing from the station. The fix is always defined as the radio station itself or as an intersection of 2 radials (or magnetic bearings) originating from 2 different stations.
A course is the track or line that an aircraft follows in relation to the ground or surface of the earth. A heading is the direction an aircraft flies through the airmass, and when the airmass is moving, unless the plane is pointed directly upwind or downwind, the heading and the course will differ by what's called the wind correction angle (WCA), or conversely, the drift angle.
The FAA currently specifies a trial-and-error method for correcting subsequent holding patterns for wind drift effect. Whenever a WCA is necessary to track (fly over a straight ground path by pointing or "crabbing" the plane as necessary into the wind) on the IB leg, that correction should be doubled on the OB leg to help compensate for the 2 uncorrected turns in the pattern. Regarding timing, they view the 1-minute IB leg 47 as the "parking space" in the sky, and it is up to the pilot to adjust his OB timing and direction so that the IB leg comes as close as possible to the 1-minute target. The FAA suggests that the first pattern after crossing the fix be flown with a 1-minute OB leg, then the IB leg be timed to find the error, or the time off 1 minute. The rule for adjustment is as follows: with an IB leg tailwind, double the IB time error and add it to the OB time used on the previous OB leg. With a headwind on the IB leg, take 1/2 the IB leg time error and subtract it from the OB leg. These crude corrections often simply help keep the pilot in the general area, and when the wind is strong, it may take 4 or 5 circuits or more for the pilot to get close to rolling out on the IB leg. Intercepting the IB leg while a minute's flight time away from the fix can even be impossible, while using the trial-and-error method. For example, when the wind speed rises to about 35% or more of the aircraft's holding speed, a direct headwind on the IB leg would require flying straight past or beyond the fix for some time to avoid the next circuit's IB leg from taking well over a minute. This is due to the wind shortening the IB leg distance traveled in 1 minute as well as 2 minutes of wind drift that the FAA doesn't allow the pilot to correct for while turning. Although it might be viewed as extending the IB leg past the fix, the additional distance added past the fix would be referred to, by the FAA and in this application, as the OB leg, since the OB leg's primary purpose is to adjust the IB leg to its ideal orientation and timing, and since the plane is actually tracking OB from the fix. The said 180.degree. correction leg cannot be derived from the trial-and-error method's "formula", nor can any correction leg whose OB angular correction is more than twice the IB correction. This is where the pilot must use his own judgment, and even a mathematical genius would have difficulty estimating the corrections to any reasonable degree of accuracy, let alone while flying the plane.
Up until now, the trial-and-error method has been the only method available for holding pattern wind compensation. There are some devices that are intended to help the pilot visualize holding pattern entries, but they have nothing to do with wind correction. Most of the devices used for any type of wind correction are related to correcting for a single-direction track along the ground, like that which a pilot needs when flying from point A to point B in a moving airmass. Examples include the well-known Dalton E-6B for solving wind triangle problems and simplified versions like that shown in U.S. Pat. No. 4,134,006 in which the "computer" is designed to work for only 1 specific airspeed for the purpose of making things simpler for the pilot of a given aircraft. There are also handheld calculators available that solve for wind triangles. However, the only use any of these would have for a pilot executing holding patterns would be to correct for the wind drift encountered on the IB leg only.
Until now, wind effect on a curved path through space has eluded all devices except the most elaborate and expensive Electronic Flight Instrument System (EFIS). I've seen this capability demonstrated only once (on TV) by an experimental EFIS, and I don't know if it made it into production.
Even this said EFIS would have definite limitations where holding patterns are concerned. To the best of my knowledge, it will make a groundtrack projection on a cathode-ray-tube display as to where the aircraft will go if the selected turn rate is held for a specified period of time, and the wind remains constant. It does this for only 1 turn at a time, and cannot be used to forecast the wind effect on a holding pattern's 2 turns separated by an OB leg. It would help a pilot capture the IB leg from the OB side by projecting the effects of adjusting his rate of turn, if he were to guess at a reasonable correction beforehand, but it would not tell him how to time his OB leg, or even what heading to fly on the OB leg. Getting reasonable corrections would still rely heavily on trial-and-error.